Animal feed for animals such as cattle and other ruminants generally include-roughage and protein supplements. Most protein supplements are grain and grain products. The roughage includes grasses and coarsely chopped plant material. The chopped plant material is generally corn that has been harvested by a forage harvester. Plants other than corn are also chopped by forage harvesters. These other plants include milo, alfalfa and occasionally wheat. The plants are chopped before maturity and while they are still green and have a high moisture content. Due to the high moisture content, the chopped plant material is stored in a silo with reduced oxygen to prevent mold formation and spoilage.
The forage harvesters that are used today to remove essentially all the plant material above the ground, coarsely chop the plant material and blow the chopped material into a transport vehicle. Some forage harvesters are pulled by large farm tractors and driven by a power take-off on the tractor. Other forage harvesters are self-propelled. Self-propelled forage harvesters as well as large tractor towed forage harvesters are capable of harvesting and chopping tens of tons of crop material per hour. For such harvesters to be efficient, a forage transporter moves beside the forage harvester and catches the chopped plant material that is discharged from the harvester's discharge blower. The forage transporters, used in large scale forage harvesting operations, are trucks with forage boxes mounted on their frames or semi-trailers. The semi-trailers are capable of transporting 20–40 tons or more of forage depending upon their length. When one trailer is filled, an empty trailer should be ready to move into position to one side of the forage harvester and receive plant material from the forage harvester as the harvester moves forward through the field and harvests more plants.
End dump trailers, which are currently employed with forage harvesters to receive and transport chopped plant material, must be unloaded where the rear axles of the trailer are horizontal. The surface that supports the rear trailer wheels must also support the trailer without sinking into the ground. When the trailer is on a slopping surface and the axle is a little higher on one side than on the other, the trailer could fall over as the front of the loaded cargo box is raised. The trailer may also fall over as it is raised if the rear wheels on one side sink into the ground. The chopped plant material with a high moisture content does not flow evenly out the rear of a cargo box as the front of the box is raised. Uneven shifting of the material in a cargo box may shift the center of gravity and contribute to a trailer falling over on its side during unloading.
Truck drivers have been known to rush back to the field to be reloaded before an end dump cargo container is lowered to its normal transport position. In some cases the cargo container is only raised a few inches and no harm is done. In other cases the front of the cargo container contacts an obstruction such as a power line.